Skip tests if optional packages not installed. Addresses #263
Skip tests if optional packages not installed.
Addresses #263

--HG--
extra : convert_revision : svn%3A3ed01bd8-26fb-0310-9e4c-ca1a4053419f/networkx/trunk%401437

ghdk/networkx,RMKD/networkx,dmoliveira/networkx,RMKD/networkx,kai5263499/networkx,ghdk/networkx,dmoliveira/networkx,SanketDG/networkx,kernc/networkx,jni/networkx,sharifulgeo/networkx,ionanrozenfeld/networkx,farhaanbukhsh/networkx,bzero/networkx,harlowja/networkx,JamesClough/networkx,Sixshaman/networkx,jcurbelo/networkx,debsankha/networkx,jtorrents/networkx,bzero/networkx,jakevdp/networkx,aureooms/networkx,debsankha/networkx,farhaanbukhsh/networkx,harlowja/networkx,jtorrents/networkx,aureooms/networkx,ghdk/networkx,jakevdp/networkx,goulu/networkx,nathania/networkx,nathania/networkx,dhimmel/networkx,wasade/networkx,ltiao/networkx,michaelpacer/networkx,debsankha/networkx,dmoliveira/networkx,bzero/networkx,aureooms/networkx,farhaanbukhsh/networkx,dhimmel/networkx,kai5263499/networkx,blublud/networkx,sharifulgeo/networkx,kai5263499/networkx,OrkoHunter/networkx,sharifulgeo/networkx,jni/networkx,ionanrozenfeld/networkx,andnovar/networkx,yashu-seth/networkx,nathania/networkx,blublud/networkx,blublud/networkx,jfinkels/networkx,harlowja/networkx,kernc/networkx,tmilicic/networkx,jakevdp/networkx,ionanrozenfeld/networkx,jni/networkx,chrisnatali/networkx,dhimmel/networkx,cmtm/networkx,kernc/networkx,beni55/networkx,NvanAdrichem/networkx,chrisnatali/networkx,RMKD/networkx,chrisnatali/networkx